The present invention relates to a signal receiver which receives an optical or electric digital signal, amplifies it by use of a receiving amplifier and then determines its logical level by use of a comparator.
In this kind of signal receiver heretofore employed, a DC blocking capacitor is provided at the input side of the receiving amplifier when the DC component of an input signal varies great by or when the drift of, for example, a photoelectric converter at the input side is large. FIG. 1 shows a prior art example of the signal receiver of the type receiving an optical digital signal. An input optical signal Pin (row A in FIG. 2) is applied to a photoelectric converter 11, in which it is converted by a photodiode 1a to an electric signal, which is amplified by an amplifier 1b. The amplified signal is provided via a DC blocking capacitor 12 to an inverting amplifier 13, in which it is further amplified, and the amplified output (row B in FIG. 2) is provided at one input terminal of a comparator 14. The DC blocking capacitor 12 and the inverting amplifier 13 constitute a receiving amplifier 15.
As shown at row B in FIG. 2 the output Vo of the receiving amplifier 15 undergoes a transient phenomenon immediately after the arrival of an input signal, under the influence of a time constant circuit which is defined by the capacitance of the DC blocking capacitor 12 and the input resistance of the inverting amplifier 13. The output Vo of the receiving amplifier 15 is compared with a threshold value V.sub.T in the comparator 14, from which is provided a signal (row C in FIG. 2) which goes high or low depending on whether the output Vo is smaller or greater than the threshold value V.sub.T.
Assuming that the threshold voltage V.sub.T of the comparator 14 is set to a low level shown at row B in FIG. 2 and, as shown in row A of FIG. 2, a second input signal p of an amplitude appreciably smaller than that of a first input signal S is applied in the OFF period of the first input signal S of an ordinary amplitude, then some of the second input signal pulses p, for example, two pulses at the right-hand side, cannot be detected by the comparator 14 as shown in row C of FIG. 2. If the threshold voltage V.sub.T is set to a high level as indicated by V.sub.T ' at row B in FIG. 2, the comparator output remains high-level for a certain period of time after arrival of an input signal as shown at row D in FIG. 2 and no signal detection is possible during this time. Thus, the conventional signal receiver is defective in that some of the second digital signals p of a small amplitude, received during the OFF period of the first digital signal S of a large amplitude, are not detected due to the aforementioned transient phenomenon of the receiving amplifier.